Subgroup (Rational Subgrouping)
Small group of observations collected under conditions as similar as possible — the base unit of X̄-R and X̄-S charts.
Also known as: rational subgroup, subgroup
A subgroup (rational subgroup) collects several observations that share, as much as possible, the same conditions. The within-subgroup variation supplies the σ estimate used for control limits; the between-subgroup variation is interpreted as a potential special cause.
- Typical subgroup size n = 4 or 5 (X̄-R for n ≤ 10, X̄-S for n > 10)
- Examples: 5 consecutive parts from one cavity; 4 measurements within one minute; 5 parallel readings of the same feature
- No-grouping case (n = 1) → use an I-MR chart
See also
Used in
In the Algorithm Lab
Sources
- Shewhart, W. A. — Economic Control of Quality of Manufactured Product, 1931
- Montgomery, D. C. — Introduction to Statistical Quality Control, 8th Edition, Chapter 5
- Wheeler, D. J. — Advanced Topics in Statistical Process Control, 2nd Edition, SPC Press